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March, 2019 Perfecting Diffusion Bonding By Jeff Elliott F


or many years, diffusion bond- ing has been utilized to join high-strength and refractory


metals that are either difficult or impossible to weld by other means. The process, which involves applying high temperature and pressure to similar or dissimilar metals mated together in a hot press, causes the atoms on solid metallic surfaces to intersperse and bond. Unlike traditional brazing tech-


niques, the resulting bond exhibits the strength and temperature-resist- ance of the base metals. It also elim- inates the need for filler material that affects the final weight and dimensions of the mated metals. Despite its benefits, the use of


diffusion bonding has been limited by more practical considerations. Spec - ific ally, the size limitation of the fur- nace chamber, as well as limits to the amount and uniformity of the pres- sure applied across the entire surface area of the part. Run times are also lengthy, often lasting a full day. Advances in high-vacuum hot


presses used for diffusion bonding are eliminating many of those con- straints. More advanced equipment now provides superior pressure con- trol, feedback from embedded pres- sure transducers, physical ink tests that show variations in pressure across the surface and rapid cooling systems to improve the bond, increase yields and significantly


PVA TePla’s advanced diffusion bonding equipment offers


feedback from embedded pressure transducers, physical ink tests and the ability to rapidly cool material.


Today, it is being used for new


applications that range from turbine blades to medical devices, heat


molybdenum to aluminum. The process is now also being


utilized for a revolutionary additive manufacturing process called lami- nated object manufacturing (LOM). In this approach, thin 0.04 to 0.08 in. (1 to 2 mm) sheets of metal are bond- ed in what is essentially an additive process. The layered sheets can be laser


cut. When combined together they can create cooling channels to dissi- pate heat. The final “sandwich,” with all its layers, can then be machined using traditional CNC turning and milling equipment.


Heat Treatment Furnaces Because diffusion bonding is a


product of heat and pressure, the heating elements and integrated hydraulic press play a key role in the quality of the final bond. For the atoms of two solid, metallic surfaces to intersperse, they typically must be at approximately 50 to 70 percent of the absolute melting temperature of the materials. To achieve these tem- peratures, the surfaces are heated either in a furnace or by electrical resistance to temperatures as high as 2,552°F (1,400°C). The pressure is applied by a


hydraulic press or dead weights. Because the two mating pieces must be in intimate contact with each other, fixtures are often used. Once clamped, pressure and heat are applied to the components, usually for many hours. Since oxidation can also affect bonding, most heat treat- ment furnaces operate under a high vacuum.


More Precise Controls While these are common ele-


ments of the process, it has been dif- ficult to control each factor precisely. In the case of the pressure applied,


increase cycle time. This has ramifications for an


increasing number of industries. Diffusion bonding is already used to create intricate forms for the elec- tronics, aerospace, and nuclear industries for items that include fuselages, actuator fittings, landing gear trunnions, nacelle frames, and nuclear control rods.


exchangers and even lithium batter- ies. Typical materials utilized in these products that are welded by diffusion bonding include stainless steel, titanium, zirconium, berylli- um, high-alloyed aluminum, Inconel, and tungsten. The process is also used to weld dissimilar metals like copper to titanium, copper to alu- minum, copper to tungsten, and even


for example, integrated single-cylin- der hydraulic presses can apply a consistent, measurable amount of force, but this provides very little control over large parts with more complex geometries. To compensate, thick graphite


pressing plates 10 to 15 in. (25.4 to 38.1 cm) must be used to mate the layers of metal together at a more consistent pressure. Unfortunately, this takes up furnace space, while adding to the time to heat up the sur- faces of the metals. Today, however, manufactur-


ers, such as PVA TePla of Corona, California, offer multi-cylinder sys- tems with large pressing plates that can accommodate a variety of parts. The largest, the company’s MOV 853 HP, can process parts as large as 3 x 4.3 ft (0.9 x 1.3m) in size, which is quite large for diffusion bonding. The pressing force is 4,000 kN. By controlling each cylinder


independently, the integrated press provides extremely consistent pres- sure across the entire surface. The MOV also comes with built-in pres- sure transducers along the bottom of the pressing plate. Based on the readings, the individual hydraulic cylinders can be adjusted to achieve uniformity over large areas. PVA TePla has optimized a


physical ink test method that can be performed to identify areas on the part where uneven pressure is being applied. For greater temperature unifor-


mity, the MOV system utilizes six heaters for temperature uniformity


Specialized furnaces with integrated presses


provide superior control of pressure and temperature to create better diffusion


bonds when joining similar or dissimilar metals.


within the chamber, instead of the usual one or two, for a maximum operating temperature up to 2,552°F (1,400°C). Rapid cooling quickly brings temperatures down so parts can be removed in about half the time, without risk of cracking or other damage. With superior temperature con-


trol and multiple hydraulic cylinders in the press, much thinner fixturing plates can be used, freeing up space in the furnace and allowing for increased cycle times, due to faster heating of the surfaces to the desired temperatures.


Laminated Object Manufacturing


Today, the diffusion bonding


process is being used for a new addi- tive technology, called laminated object manufacturing (LOM). This has tremendous potential applica- tions for conformal cooling. Parts are designed using tradi-


tional 3D CAD modeling programs, then divided into two-layer sections that equal the thickness of each sheet of metal. The processing time is


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